We have to stop thinking about problems so linearly -- it's not "solve only the worst one first", because we'll forever find reasons to not try and solve that one, and we'll throw up our hands.
Like, we're well aware animal agriculture is a huge environmental impact. But getting everyone to go vegetarian before we start thinking about any other emissions source is a recipe for inaction. We're going to have to make progress, little by little, on all of these things.